A chromatograph mass spectrometer, which consists of the combination of a high-speed liquid chromatograph (LC) or gas chromatograph (GC) and a mass spectrometer (MS), is capable of repeating a mass analysis over a predetermined measurement mass range (specifically, a mass-to-charge ratio range over which the mass analysis is to be performed) to obtain a series of mass spectra of various components of a sample eluted from a column of the LC of GC with the lapse of time. An ion detector of the mass spectrometer typically includes a secondary electron multiplier combined with a conversion dynode, microchannel plate or similar element.
The ion detector and other elements in the subsequent stages, such as a current/voltage converter or amplifier, include electrical circuits, which inevitably produce electrical noise and may also receive external noise. Therefore, the detection signal obtained during the mass scan operation will contain an electrical noise signal superimposed on a signal produced by the ions originating from the sample. Given these factors, conventional mass spectrometers perform a noise-removing process, which includes measuring a noise component due to the aforementioned electrical factors before the measurement of a target sample, and then subtracting the noise information obtained by the noise measurement from the mass spectrum information of the target sample.
Mass spectrometers perform an averaging process on a set of data obtained in two or more mass scan cycles to stabilize the shape of mass spectra, and some of these apparatuses can change the number of mass scan cycles for the averaging process during the measurement according to a change in the analysis conditions. For example, the apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2001-99821 can switch its operational mode between the positive-ion measurement mode and the negative-ion measurement mode for each mass scan cycle or between the normal mass analysis and the MS/MS analysis including a dissociating operation. Changing the number of mass scan cycles creates a different state of noise. Therefore, the aforementioned noise-removing process should be preceded by a preprocess in which the noise information obtained by measuring the noise component is appropriately processed by a statistical method that takes into account the number of mass scan cycles.
However, the level of the electrical noise from the circuits of the ion detector, amplifier and other elements usually changes with time since the state of this noise is sensitive to temperature and other factors. Therefore, in some cases it is impossible to appropriately remove the noise by performing the noise-removing process using the noise information obtained by the preliminary measurement of the noise before the measurement of the target sample.
One known method for avoiding these problems is to perform a noise-removing process using additional noise information obtained by repeatedly measuring the noise component at specific intervals of time during the measurement of the target sample as well as before the same measurement. However, this technique cannot consistently provide a desired noise-removing effect since there is a certain time-gap between the measurement of the target sample and that of the noise component; if the electrical noise has increased during the measurement of the target sample, the time-gap may prevent this increase in the noise from being correctly reflected in the noise information.
The present invention has been developed in view of these problems. Its objective is to provide a method of processing mass analysis data capable of accurately creating mass spectra by properly removing electrical noise from an ion detector, amplifier or other elements, and also a mass spectrometer capable of such a data processing.